Transistors may be formed by processes wherein their semiconducting layer, and in many cases, other layers is deposited from solution. The resulting transistors are called thin-film transistors. When an organic semiconductor is used in the semiconducting layer, the device is often described as an organic thin film transistor (OTFT).
Various arrangements for OTFTs are known. One device, a top-gate thin-film transistor, comprises source and drain electrodes with a semiconducting layer disposed therebetween in a channel region, a gate electrode disposed over the semiconducting layer and a layer of insulating material disposed between the gate electrode and the semiconductor in the channel region.
The conductivity of the channel region can be altered by the application of a voltage at the gate. In this way the transistor can be switched on and off using an applied gate voltage. The drain current that is achievable for a given voltage is dependent on the mobility of the charge carriers in the organic semiconductor in the active region of the transistor, namely the channel region between the source and drain electrodes. In order to achieve high drain currents with low operational voltages, organic thin film transistors must have an organic semiconducting layer which has highly mobile charge carriers in the channel region and an efficient means to inject charge from the electrode to the organic semiconducting layer.
In short channel length devices contact resistance can contribute a significant proportion to the total channel resistance in the device. The higher the contact resistance in the device, the higher the proportion of the applied voltage is dropped across the source and drain contacts and, as a result, the lower the bias across the channel region is achieved. A high contact resistance has the effect of a much lower current level being extracted from the device due to the lower bias applied across the channel region, and hence lower device mobility.
There are a number of different ways to reduce or minimise contact resistance. One approach is to dope the semi-conducting layer, for example, with a p-dopant. Lee, Jae-Hyun et al. in Applied Physics Letters 98, 173303 (2011) and Qi, Yabing et al. in J. Am. Chem. Soc. 2009 131 12530-12531, for example, both disclose the doping of organic semiconducting layers with molybdenum tris-[1,2-bis(trifluoromethyl)ethane-1,2-dithiolene] (Mo(tfd)3). Mo(tfd)3 has a low LUMO level of about 5.59 eV making charge transfer from the HOMO of an organic semiconductor to Mo(tfd)3 energetically favourable in many cases. It is also recognised that Mo(tfd)3 can be more uniformly distributed throughout organic semiconductors than metal oxide dopants resulting in higher charge generation efficiency.
Tiwari, S. P. et al. in Organic Electronics 11 (2010) 860-863 discloses the fabrication and characterisation of OFETs wherein a 10 nm co-evaporated layer of Mo(tfd)3 and pentacene is deposited under the metal (Au) electrodes and over a pentacene semiconducting layer. Coevaporation is carried out through a shadow mask to define the source/drain electrodes. Tiwari, S. P. et al. hypothesise that selective doping near the electrode interface decreases interface resistance due to a reduction in the energy barrier height for carrier injection form the metal electrode to the semiconductor due to band bending associated with doping at the interface. Thus in the method of Tiwari, S. P. et al. the Mo(tfd)3 migrates into and dopes the organic semiconducting layer as in Lee, Jau-Hyun and Qi, Yabing described above.